In recent years, it has become increasingly important to reduce or eliminate sulfur oxides from gases being introduced into the atmosphere. Typical of such gases are the flue gases generated by the combustion of high-sulfur-content fossil fuels.
Although many process for removing sulfur oxides from gases discharged to the atmosphere have evolved over the past few years, one of the most widely accepted and employed techniques is the wet scrubbing of sulfur dioxide-containing flue gases with aqueous ammoniacal solutions. A variety of processes and schemes have been suggested for the efficient and economical removal of sulfur oxides from flue gases utilizing this primary technique. A recent report by the Tennessee Valley Authority (TVA) entitled "Sulfur Oxide Removal From Power Plant Stack Gas; Conceptual Design and Cost Study Series, Study No. 3, Ammonia Scrubbing (Bulletin Y-13), Oct. 1970," has thoroughly discussed this subject and presents numerous process alternatives. Demonstration of the scrubbing process using ammoniacal solutions has been reported by the TVA in cooperation with the Environmental Protection Agency. Results of this work were presented by Hollinden et al. at the Flue Gas Desulfurization Symposium, New Orleans, La., May 14-17, 1973 in a paper entitled "Removal of Sulfur Dioxide From Stack Gases by Scrubbing With Ammoniacal Solutions, Pilot Scale Studies at TVA."
The reaction of sulfur dioxide, water, and ammonia is a well-known reaction forming a solid precipitate either in the vapor phase (see U.S. Pat. Nos. 3,579,296 and 3,928,536) or in the liquid phase of the typical scrubbing operation. Although these previously suggested processes adequately reduce the gaseous sulfur dioxide content in the flue gases being treated, such processes can also yield fine particles of solid ammonium sulfite, ammonium bisulfite, or ammonium sulfate as a result of the vapor-liquid contacting. Such result is not only troublesome, but highly undesirable in that it results in the ultimate discharge of such solid particulate matter to the atmosphere and can readily be seen as a "plume" at the top of the stack. In this connection, U.S. Pat. No. 3,843,789 addresses itself to the controlled scrubbing of sulfur oxides from flue gases using ammoniacal solutions in order to prevent solid particulate formation in the vapor phase either before, within, or after the scrubber. While such procedure is operable and does effect the elimination of "plume" in certain instances, there are other cases when the techniques of this patent do not eliminate the formation of plume.